Optimizations for so-called “small data” scenarios—that is, scenarios in which only very few data are transferred in a given time unit—are being discussed in 3rd Generation Partnership Project (3GPP). Such scenarios may include infrequent transfer of a small number of data packets, although the exact data amount does not need to be defined explicitly. Example use cases are sensor or actuator devices which transmit or receive small amounts of data regularly but not continuously (e.g., a sensor transmitting its measurement report once per time unit, such as every ten minutes).
Current communication systems are typically not optimized for this type of small data transfer, and a large overhead is thus involved in “small data” scenarios. One possible optimization has been proposed by 3GPP, in which the NAS protocol is used to transfer small data amounts, see 3GPP SA WG2 Temporary Document S2-113826, “Small Data Transfer (E-UTRAN): Use of pre-established NAS security context to transfer the IP packet as NAS signalling without establishing RRC security”. The main concept of data-over-NAS will now be described with reference to FIG. 1, which shows an example case of uplink transmission in a 3GPP Long Term Evolution (LTE) communication system.
FIG. 1 illustrates the principle of the data-over-NAS transmission scheme in the uplink. Specifically, FIG. 1 shows a communication network 100 comprising a terminal, or User Equipment (UE) 101, a base station (such as an (evolved) node B, (e)NB) 1011 and a Mobility Management Entity (MME) 1012, and the associated NAS signalling. Initially, the basic random access and Radio Resource Control (RRC) connection setup procedures may be used as defined in the 3GPP specifications. An RRC Connection Setup Complete message carries the data packet(s) (denoted as “NAS Uplink (UL) DATA”) from the UE 101 to the base station 1011. The NAS data are then transferred from the base station 1011 to the MME 1012 in an Initial UE Message over the S1 interface. The S1 interface is the control plane interface between the base station 1011 and the MME 1012. If there is some follow-up downlink data within a short period of time, the follow-up data (denoted as “NAS Downlink (DL) DATA”) can be sent over NAS in a Downlink NAS Transport message from the MME 1012 to the base station 1011 and from there via a DL Information Transfer message to the UE 101.
After a given period of time, the S1 connection and the RRC connection may be released as shown in FIG. 1. Then, between the MME 1012 and a Serving Gateway (SGW, not shown), some special packet delivery mechanism is needed (i.e., to send some data packets over the S11 interface, wherein the S11 interface stands for the interface between the MME 1012 and the SGW in the Evolved Packet System, EPS).
FIG. 2 illustrates the principle of the data-over-NAS transmission scheme in the downlink. FIG. 2 again shows the communication network 100 comprising the UE 101, the base station 1011 and the MME 1012 and the associated NAS signalling. In the case of a DL packet, if the S1 interface and RRC connection are not established for the UE 101, the MME 1012 first needs to perform paging which triggers the UE 101 to send a Service request message (in the RRC Connection Setup Complete message shown in FIG. 2), which message is forwarded by the base station 1011 to the MME 1012 in an Initial UE Message.
When the MME 1012 receives the Service request, the MME 1012 may use a priori information pertaining to the UE 101 or the associated subscription to determine that no bearer setup is needed. Based on that determination, the MME 1012 can send the downlink small data as “NAS DL DATA” in a Downlink NAS Transport message, which data may be forwarded from the base station 1011 to the UE 101 in a DL Information Transfer message.
The signalling illustrated in FIG. 2 may possibly be followed by uplink data transmission as well (see messages UL Information Transfer and Uplink NAS Transport in FIG. 2). After a certain period of time, the MME 1012 may again release the S1 connection and RRC connection.
The data-over-NAS transmission scheme discussed above with reference to FIGS. 1 and 2 has certain advantages. The signalling load on both the MME 1011 and the UE 101 can be reduced. For the UE 101, a consequence resides in the fact that the energy consumption will be reduced. This is especially important for small devices, e.g., sensors or actuators that may run on a limited power supply (e.g., on battery).
The data-over-NAS transmission scheme requires a special behaviour both in the UE 101 and the network, and hence its use has to be negotiated in advance. This can be done, for example, during the Attach or connection setup procedures.
It has been considered to use the data-over-NAS transmission scheme for a single bearer in a dynamic fashion when the use of NAS is expected to reduce overhead based on some prediction of the traffic pattern. In such a case the (same) bearer is “switched” between the data-over-NAS transmission scheme and the regular bearer approach without any explicit control signalling. The switching might, for example, be subject to the following rules:                The UE 101 applies the data-over-NAS transmission scheme for an uplink packet when a local application needs to send just one data packet (and this uplink packet should not trigger multiple downlink packets).        The MME 1012 uses information of the subscription to determine whether a packet sent over NAS in the uplink should result in the establishment of a regular bearer or not.        For a downlink packet, the Downlink Data Notification acknowledgement (ACK) sent by the MME 1012 to the SGW is extended to inform the SGW that the packet has been delivered (or that the normal Network Initiated Service Request procedure has been triggered).        If the SGW has not received a Downlink Data Notification ACK indicating that the normal Network Initiated Service Request procedure has been triggered, when a second downlink data packet arrives in the SGW, the SGW sends a new Downlink Data Notification with that data packet appended to the MME 1012. If the SGW receives multiple data packets, the SGW can use the Downlink Data Notification to request the MME 1012 to perform the normal Network Initiated Service Request procedure.        The SGW monitors whether subsequent downlink data packets have arrived after an initial downlink packet for the delivery to the UE 101 and whether the total size of these data packets is greater than the value configured by the network operator's policy or by the subscription. If this is the case, the SGW sends the Downlink Data Notification to request the establishment of the S1 bearer(s).        
It has been found that there exist difficulties with the dynamic switching between the data-over-NAS transmission scheme and the regular bearer mechanism. Firstly, relying on a local application in the UE 101 to select the type of transmission has several disadvantages:                The application has to have information on the transmission scheme and must be able to select between different Application Programming Interfaces (APIs) for different transmissions. Hence, the application has to be adapted for use in an NAS-enabled network, which means that regular off-the-shelf applications (e.g., those used on laptops) cannot be used.        For an uplink packet in the UE 101, it may turn out to be difficult in practice to determine by the application, on a packet-by-packet basis, how many packets are to follow. Here, it may be too restrictive to put requirements on the application itself. Furthermore, it is often difficult to predict future traffic patterns.        There is a risk if the application makes the wrong decision: if the initial data packets are sent as data-over-NAS and it frequently turns out that the UE 101 has subsequent data packets and then transitions to connected mode, the total signalling load on NAS is actually higher compared to today's procedures (i.e., when the UE 101 immediately goes to connected mode) because of the additional initial NAS signalling.        
Secondly, the algorithm in the UE 101 and in the core network to dynamically switch between the data-over-NAS and regular bearer mechanism can lead to a number of new error cases that take a lot of effort to handle. Testing, debugging and operating the communication system may become more difficult and more costly due to the uncertainty regarding which path an individual data packet in a traffic flow would take.
Thirdly, for downlink traffic, it is especially difficult to predict how many packets are to follow an initial downlink packet. The amount of buffered downlink data packets in an SGW does not give a good indication, because new downlink data packets are often the result of an uplink response to the initial downlink data packets. So the SGW has no means to easily determine whether there will be many downlink data packets following an initial downlink data packet. Furthermore, a strategy which sends the initial downlink data packet as data-over-NAS and then sets up the regular bearers for subsequent downlink data packets is prone to a large overhead, because it incurs not only the already existing idle-connected-idle signalling overhead, but also the overhead of sending the first data packet over NAS. So such a strategy may be inefficient if there are often subsequent data packets that follow the initial data packet.